Saturday, November 24, 2012

LIfe on Titan--Metabolism Inverted

Titan is Saturn’s largest moon. With an atmosphere about twice as thick as Earth, but much colder (the average temperature on the surface is around minus180 Celsius—approaching 300 below in Fahrenheit), life on as we know it on Earth would seem to be impossible on Titan. Liquid water forms the common solvent of all life on our planet, of all sizes, for plants, animals, bacteria, and everything else. On Titan, water is a rock as much as slate is on our world. As on Earth, internal heating can cause the rock to melt—but on Titan, this would mean that liquid water would flow as a cold version of lava, only to solidify again into water rock. On the surface, no life could exist that depends on liquid water.

It could be that if there is a permanent reservoir of molten water under the surface of Titan, just as there is a permanent flow of magma under the surface of the Earth. If so, liquid-water based bacteria might live below. That perhaps could be, but what interests me is the possibility of life on Titan’s surface.

It so happens that Titan has another form of liquid on its surface, lakes of liquid methane and ethane, as imaged by radar on the Cassini probe and shown in false color below:

A number of scientists think it’s possible that liquid forms of the hydrocarbons methane or ethane could serve as a solvent for the chemicals of life. It also has been proposed that this life in liquid  hydrocarbon would gain metabolic energy on Titan by breaking down more complex molecules like ethane into methane, releasing hydrogen molecules as a byproduct (as discussed in this Wikipedia Life on Titan link). 

In contrast to the most-widely accepted theories of possibilities of life on Titan, I think it would be interesting to create a story that features life there as more of a direct inversion of life on Earth. To illustrate what I mean, think of a jet airplane. For fuel, the jet carries a hydrocarbon chemically related to methane (but more complex) in liquid form in its fuel tanks. As it flies through the air, the fans of the engine capture atmospheric oxygen and combine in with the fuel in a reaction that generates a lot of heat, causing expansion of the propelled exhaust, driving the plane forward.

On Titan, a jet could fly through the atmosphere by fueling up with liquid oxygen (which would only require a small amount of refrigeration at Titan’s temperature and pressure) capturing atmospheric methane in its turbofans and combining the two for propulsion. The chemical reaction would be the same as on Earth (or nearly so) but the means by which the chemicals enter the reaction would be inverted.

So imagine plant-like creatures on Titan that ingest solid water and incorporate it into their body structure (at least in part). For them energy would come from some analog of photosynthesis, but would probably function pretty slowly, since Titan gets around 1% the sunlight Earth receives. The plant might easily produce oxygenated compounds other than water, such as hydrogen peroxide, which is also a solid at Titan’s temperatures.

Then an animal of Titan could consume the leaves of this plant and the oxygenated compounds would be digested and pushed through liquid ethane-based blood into each body cell. This sort of animal could also inhale atmospheric methane (like Earth, most of Titan’s atmosphere is nitrogen, but the next most common gas is methane), maintaining just enough body warmth to maintain the fine balance where methane is gaseous but the ethane stays liquid, doing what water does for us. Just as our blood carries oxygen and dissolved hydrocarbon (for us, glucose) which are combined in each cell to provide the energy it needs, the blood of these Titan aliens could carry both fuel (for them, methane) and oxidizer to each cell, where they would combined for energy much the same way as life on Earth, only inverted in terms of means of entry into the body system.

Carbon dioxide, the exhaled waste product of human metabolism, is a solid on the surface of Titan, but carbon monoxide is a gas and could be exhaled by Titanian life. Other waste products of metabolism could flow out with ethane “urine” or pass out in semi-solid form with the undigested body ejecta.

So life on Titan could in effect operate much as we do—they could even look virtually the same as Earth life forms—only with the means of metabolism inverted and at a much colder temperature. The lower temperature would have the effect of making chemical reactions run much more slowly. Giant tree sloths might easily outrun the speediest life form on Titan…

Note though that the temperature difference between liquid methane and ethane I referenced above is only about one degree. A human’s body easily maintains its core temperature within one degree on a regular basis, so it does not seem improbable to me at all that a complex multicellular organism could do the same on Titan. But note that a single-celled organism would be very unlikely to maintain such precise temperature control (Earth microorganisms can’t do that).

So someone who believed in random evolution of life probably would say my proposed method of metabolism is impossible for complex life, since simple life could not use it, meaning there would be no opportunity for simple life employing that mechanism to evolve to a complex form. 

But what if a story portrayed complex life existing on Titan without any single-celled organisms of the same metabolism type in existence? Would human scientists landing on such a Titan speculate the simpler life forms must have gone extinct? Or that life on Titan evolved somewhere else in the galaxy, only to be somehow transported to our Solar System? Or would they wonder if these creatures had been directly generated by some kind of creator, perhaps even the same one I would call “God”…



  1. Note that even though no life was seen by the Huygens probe from Titan's surface, there are plenty of spots on Earth where one could land without seeing any sign of life...